In the RF and microwave frequency ranges (1 MHz to over 40 GHz), devices can be characterized by their scattering (S) parameters. S-parameters provide information on device performance and can be easily measured by commercially available S-parameter measurement devices, also known as network analyzers. Network analyzers can be grouped into two categories: scalar network analyzers (SNAs) and vector network analyzers (VNAs). SNAs measure the amplitudes of the S-parameters, while VNAs measure both the amplitudes and phases of the S-parameters.
In operation, when the ports of a device under test (DUT) are connected to the ports of a network analyzer, the network analyzer applies a signal to each device port in succession and measures the amplitude and the phase (when using a VNA) of the reflected and transmitted waves to determine the S-parameters of the DUT. However, with virtually any network analyzer, there are inevitable hardware imperfections that can produce significant measurement errors if the imperfections are not accounted for in the measurements. The process of characterizing the imperfections in the network analyzer is known as calibration.
The basic premise of calibration is to use a mathematical error model of the network analyzer with a number of unknown error terms to describe all of the main error contributions. In practice, calibration involves connecting certain well-known devices, called standards, to the network analyzer, and using the resulting measurements to mathematically solve for the error terms of the error model. After calibration, the error terms can be removed from the measurement of any DUT to correct for imperfections in the network analyzer.
Most network analyzers are designed to measure the S-parameters of two-port devices, typically with coaxial or waveguide interfaces. However, as technology advances, more complicated DUTs that require multi-port S-parameter characterization are becoming common. To accommodate multi-port DUTs with a two-port network analyzer, the two-port network analyzers are often connected to programmable switch boxes that contain at least as many ports as the number of ports on the DUT. The network analyzer measures the S-parameters of the DUT between two device ports by setting the switch to couple the two switch ports connected to the two device ports to the two network analyzer ports. The remaining switch ports are terminated in the switch box.
However, the addition of a programmable switch box introduces new errors into the measurement system. To remove the switch box errors, each transmission path through the switch box (i.e., each switch port pair) must be calibrated. Thus, with N switch ports, N(N−1)/2 transmission paths must be calibrated. For example, for a 10-port DUT, in one calibration method, 45 separate two-port measurements are required to determine the error terms for the switch box. Calibrating such a large number of transmission paths is time consuming, difficult and cost-prohibitive.
Therefore, what is needed is a calibration system and method that reduces the number of full calibrations necessary to characterize a multi-port S-parameter measurement device.